1. Field of the Invention
The present invention relates to a speaker apparatus and, in particular, to a speaker apparatus which is used in a microspeaker (a small-sized or subminiature speaker) and has a dome-type vibration plate having an elliptical shape or a round-edged rectangular shape.
2. Description of the Related Art
Conventionally, a speaker apparatus, which is used as a voice output apparatus of an acoustic system such as an audio system, converts an electric signal including an audio component output from an amplifier into air vibrations to thereby reproduce and output voice data and acoustic data.
A speaker, which is classified as a dynamic-type speaker according to the operation principle, can be roughly divided into a cone-type speaker the voice coil of which is driven in the root portion (cone neck portion) of a conical-shaped vibration plate, and a dome-type speaker the voice coil of which is driven in the outer peripheral portion of a vibration plate.
The cone-type speaker is generally used as a low tone speaker having a large caliber. On the other hand, in the case of the dome-type speaker, because it is driven by a voice coil having substantially the same caliber as the vibration plate, it is used as a middle and high tone speaker having a small-caliber as vibration plate; and also, recently, the dome-type speaker is used as a microspeaker (a small-sized or subminiature speaker) which is built in a cell phone.
As a specific example of the dome-type speaker, there are known “dome speaker and a method for manufacturing the same” which is disclosed in JP-A-11-146487. According to this, there are shown a dome-type speaker in which a semi-spherically-shaped vibration plate is formed flat, and a method for assembling the neighboring portion of a voice coil in which there is formed a U-shaped groove between a dome portion composed of a vibration plate having a dome shape and an edge damper portion existing in the outer periphery of the dome portion, the voice coil is inserted into the interior of the present groove and is fixed there using an adhesive agent.
And, there are also known “speaker unit and a method for manufacturing the same” which is disclosed in JP-A-9-322289. According to this, there is shown a technique in which, in a dome-type speaker having a semispherical-shaped vibration plate main portion, a rising portion is formed in the end edge of the support portion of the vibration plate and, when placing the support portion on the stepped portion of a speaker frame, the rising portion is fitted with a vertical wall portion formed so as to be continuous with the stepped portion while sliding the rising portion in contacted with the vertical wall portion.
As a microspeaker (a small-sized or subminiature speaker) to be built in a cell phone, there is often used the above-mentioned dome-type speaker; however, in order to expand a frequency range and enhance voice quality, in a narrow speaker installation area, there must be installed a speaker having a caliber which is as large as possible.
Therefore, as a method for installing a speaker having a caliber as large as possible in a limited space, there can be expected a method in which a vibration plate is formed in an elliptical shape or in a round-edged rectangular shape.
As an example in which a vibration plate is formed in an elliptical shape or in a round-edged rectangular shape, there is known a cone-type speaker designed for carriage on a car (for example, “an elliptical-shaped speaker” which is disclosed in JP-A-2000-333294).
In the case of the cone-type speaker with an elliptical-shaped vibration plate or a round-edged rectangular-shaped vibration plate as disclosed in the above-cited publication, since the vibration plate is situated outside a voice coil, the voice coil (and voice coil bobbin) may be circular in shape.
However, in the case of the dome-type speaker, when a vibration plate is formed elliptical or round-edged rectangular in shape, a voice coil must be formed elliptical or round-edged rectangular in shape.
Referring again to the speaker apparatus shown in “a dome speaker and a method for manufacturing the same” disclosed in the above-cited publication JP-A-11-146487, there is suggested a structure in which a substantially spherical shaped vibration plate is formed flat; and, even in case where the substantially spherical shaped vibration plate is formed flat, since the periphery of the vibration plate is circular in shape, there is employed a dome-type structure having a circular-shaped caliber, in which there is used a cylindrical-shaped voice coil.
On the other hand, a method for manufacturing an elliptical-shaped voice coil with a voice coil wound on a voice coil bobbin is disclosed in JP-A-5-344590.
However, the manufacturing method disclosed in the above-cited publication requires a specific jig tool when assembling the voice coil. And, in the case of an air core voice coil excluding a voice coil bobbin, when manufacturing the air core voice coil, it causes a spring back phenomenon, which raises a problem that the voice coil is easy to deform in shape.
Also, in the case of the elliptical-shaped or round-edged rectangular-shaped speaker apparatus, not only the center positions of the vibration plate, voice coil and magnetic gap must be made to coincide with each other, but also the directions of these components must be made to coincide with each other in order to prevent their axial-direction rotational movements from shifting from each other.
Further, as disclosed in the above-cited publication JP-A-9-322289, in the structure in which the rising portion is formed in the end edge of the support portion of the vibration plate to thereby position the frame and vibration plate, a stepped portion is formed on the entire periphery of the frame and the stepped-portion-formed portion of the frame increases in thickness by an amount corresponding to such provision of the stepped portion, which increases the weight of the frame.